1. Field of the Invention
This invention relates to touch sensor pads, and in particular, to determining coordinates of simultaneous touches on a touch sensor pad.
2. Statement of the Problem
It is common to use touch sensor pads to receive information from a user. For example, it is common for retail stores to have a touch sensor pad that captures a buyer's signature as the buyer moves a stylus over the touch sensor pad. Touch sensor pads are also used on personal digital assistants (PDAs), tablet PCs, notebook computers and other computing devices for inputting data into the devices. For example, a PDA or tablet PC may allow a user to write or draw on a touch screen of the device using a stylus, and the touch sensor pad captures information regarding the user input. Other devices may display a graphical user interface (GUI) on a touch sensor screen, and allow the user to manipulate a software application by touching one or more objects displayed on the touch sensor screen.
One common type of touch sensor pad is constructed from two sheets of conductive materials facing one another other and each having a resistive coating on one side. FIG. 1 illustrates a prior art touch sensor pad 100. A first resistive sheet 110 has terminals 112 and 114 disposed on top and bottom ends of first resistive sheet 110. Terminal 112 is connected to a controller 130 through wire 150, and terminal 114 is connected to controller 130 through wire 152.
A second resistive sheet 120 has terminals 122 and 124 disposed on left and right ends of second resistive sheet 120. Terminal 122 is connected to controller 130 through wire 140, and terminal 124 is connected to controller 130 through wire 142. The conductive surfaces of first resistive sheet 110 and second resistive sheet 120 are disposed facing each other and spaced apart by regularly spaced insulator dots or spacers (not shown), which keep the sheets separated at rest. When the surface of touch sensor pad 100 is touched, the location of the touch point is determined by controller 130.
To determine the location of a touch point, a voltage is applied between terminals 112 and 114 to set up a voltage gradient along linearly resistive first resistive sheet 110. The location where a user touches touch sensor pad 100 causes a connection between first resistive layer 110 and second resistive sheet 120. As a result of the connection, the voltage potential of second resistive sheet 120 rises to the voltage at the touch point on first resistive sheet 110, and can be read from terminals 122 and 124 by controller 130. The measured voltage determines a coordinate in a first dimension (e.g., a y coordinate). The process is then repeated by applying a voltage between terminals 122 and 124 to set up a voltage gradient across second resistive sheet 120, and first resistive sheet 110 is left un-energized. The voltage potential on first resistive layer 110 rises to the voltage at the touch point on the second resistive sheet 120, and can be read from terminals 112 and 114 by controller 130 to determine a coordinate in a second dimension (e.g., an x coordinate).
It is a problem to have a touch sensor pad 100 that can resolve the location of multiple simultaneous touches with a high degree of accuracy. FIG. 2 illustrates a close up view of first resistive sheet 110 of FIG. 1. Referring to FIG. 2, a voltage gradient 210 is applied between terminals 112 and 114. Assume a user simultaneously touches touch sensor pad 100 at touch point 220 and touch point 222. Because both touch points 220 and 222 will make a connection between first resistive sheet 110 and second resistive sheet 120 (not visible in FIG. 2), the voltage potential of second resistive sheet 120 will rise to an average of the voltage at touch points 220 and 222. As a result, controller 130 will determine a single touch point 230 which is an average of the positions of touch points 220 and 222. This results in an input error determined by touch sensor pad 100.
The above described input error can occur when a user intentionally touches two points simultaneously, or when a user inadvertently touches touch sensor pad 100 at two points simultaneously, such as with a stylus and their palm. Additionally, it is a problem to determine which touch points are coming from a relatively sharp object (e.g., a pen, pencil, stylus, etc.), a finger, or a resting hand. Determining the type of object contacting touch sensor pad 100 may be useful in determining an action to perform based on the type of object contacting touch sensor pad 100, such as a button press, handwriting recognition, typing, etc.